The present invention relates generally to an optical head for detecting defocuses of a target to be maasured using a focusing optical system. The optical head of the invention is applicable, as an optical pickup head, to an information recording and/or reproducing apparatus using optical recording medium as a target. The invention is also applicable to a focus adjusting mechanism in an optical position sensor, a microscope or the like.
In an optical information recording/reproducing apparatus, it is generally indispensable to fabricate an optical pickup head in small size and lightweight for obtaining rapid access. For this purpose, recently, it has been proposed to integrate an optical pickup head by employing a waveguide element.
An exemplary optical pickup head utilizing a collector/grating coupler is disclosed in a thesis entitled "Collector/Grating Coupler of Twin-Focus Type for Optical Pickup", published at General Meeting in 1986 of Optic-Electric Wave Department of Electro-communication Society (hereinafter referred to as a first reference). The arrangement disclosed therein is such that light emitted from a semiconductor laser is collimated by a collimating lens and then passes through a collector/grating coupler formed on an optical waveguide layer which, in turn, is formed on a transparent substrate of the waveguide element. Thereafter, the collimated light is focused on an optical disk through an objective lens. Light reflected from the optical disk passes through the objective lens and is again incident on the collector/grating coupler which can diffract a fraction of incident light so as to couple it with the waveguide layer, so that the diffracted light can be waveguided within the waveguide layer. The collector/grating coupler can also split the diffracted light into two collected light beams. Two pairs of photodetectors are integrated with the waveguide layer at two positions so that when the optical system is in focus with respect to the optical disk, one of the waveguided beams is collected at a midpoint between the photodetectors .fo one pair, while the other waveguided beam is collected at a midpoint between the photodetectors of the other pair. The photodetectors of each pair are aligned to each other in a direction transverse to the waveguide direction. Each photodetector can generate an output signal. The magnitude of the signal corresponds to the amount of light received thereon. Based on the output signals of the photodetectors, a focus error signal is obtained using a Foucault method, while a tracking signal is obtained using a push-pull method.
Japanese Patent Laying Open (KOKAI) No. 63-71946 (hereinafter referred to as a second reference) discloses another arrangement of an optical pickup head in which a grating coupler, a grating beam splitter, and several photodetectors, are integrated with a waveguide element. The arrangement disclosed therein is such that a light beam emitted from a semiconductor laser diverges through the grating coupler formed on a waveguide layer which, in turn, is formed on a transparent substrate of the waveguide element. The beam is then focused on an optical disk. through an objective lens. The light reflected back from the optical disk is converted to collected light by the objective lens and is then incident on the grating coupler which diffract a fraction of the collected light so as to couple it with the waveguide layer, so that thc collected light can be waveguided within the waveguide layer. The waveguided light is then split into two collected beams deflected in different directions. Two pairs of photodetectors are disposed in the same manner as those in the first reference. A focus error signal, a tracking signal, and data signals can be obtained in the same manner as described above.
Another type of optical pickup head is disclosed in a thesis entitled "Integrated-Optic Disk Pick-up Device", published by IEEE, Journal of Lightwave Technology, Vol. LT-4, No. 7, July 1986 (hereinafter referred to as a third reference). The optical head disclosed therein comprises a waveguide element having a buffer layer and an optical waveguide layer which are successively formed on the Silicone substrate (hereinafter referred to as Si-substrate). A grating beam splitter and a collector/grating coupler are respectively formed on the waveguide layer. Further, a semiconductor laser is coupled with the end face of the Si-substrate so that a laser beam emitted therefrom is waveguided by the waveguide layer. After the waveguided beam diverges through the grating beam splitter, it is diffracted by the collector/grating coupler out of the waveguide element and is then focused on an optical disk. After the reflected light from the optical disk is again incident on the collector/grating coupler, a fraction thereof is converted into waveguided light. Thereafter, the grating beam splitter diffracts and splits the collected-waveguided light into two waveguided beams which are detected by two pairs of photodetectors in the same manner as those shown in the aforementioned references. Accordingly, A focus error signal, a tracking signal, and data signals can be obtained in the same manner as described above.
In the arrangement shown in the first reference, the focus error signal is obtained using the Foucault method, as mentioned above. In this case, in order to fabricate the optical pickup head small, a focal distance of the grating coupler has to be shortened so as to decrease the path of the detecting optical system. However, the more the focal distance is shortened, the more the diameter of focused spots of the waveguided beams at respective midpoints between the two photodetectors of the two pairs becomes small. This causes the constituent elements to be positioned in place with high accuracy so that when the optical disk is positioned at a predetermined standard position on the optical axis, the focused spots of the waveguided beams are each positioned at a midpoint between the corresponding two photodetectors.
Further, the more the path of the detecting optical system is shortened, the more the focus-error detection sensitivity, which corresponds to the ratio of the magnitude of the focus error signal to the amount of the focus error, becomes worse. In order to eliminate such a problem, it becomes necessary to decrease the gaps in size between the two photodetectors of pairs in proportion to the shortening of the detecting system path. However, there is a technical limitation, in fabricating the photodetectors, to decreasing the size of the gap between the two adjacent photodetectors placed side by side with each other. For example, if PIN type photodiodes are fabricated on the Si-substrate, a gap of more than about 10 .mu.m occurs between the two adjacent photodiodes of each pair.
Accordingly, it has been difficult to fabricate an optical head which is small and lightweight without deteriorating detection-sensitivity to a focus error of an optical system in relation to an object.
The above-mentioned circumstances are similarly applied to the cases of the second and third references in which a focus error signal is obtained using the same Foucault method.
In the case of the third reference, particularly, for the purpose of positioning the focused spots of the collected-waveguided beams at respective midpoints of the photodetectors, it is necessary to precisely adjust a position of the semiconductor laser with respect to the Si-substrate by moving it in a direction parallel to the plane surface of the Si-substrate. However, it is very difficult to carry out such a precise adjustment while maintaining coupling of light with the waveguide layer.